Mandb does not include guix package man pages

[Pan Xie <xiepan@skyguard.com.cn>]

Hello

I find this issue on both GuixSD and guix package manager on ArchLinux. 
The problem is `man -k' can not find manpages installed by guix. I 
believe the issue is caused by `mandb' does not include guix packages' 
man pages when generating its index database.

Confirm sbcl is installed:
#+begin_example
[0]:root@guix03:~# guix package -I | grep sbcl
sbcl                  2.4.0           out 
/gnu/store/093cjg5472s8j8hjzi2as61cs4w3lwrf-sbcl-2.4.0
#+end_example

"man -k" can't find sbcl from its index database. But `manpath' has
the correct setting, thus `man -w' can locate the man page file
#+begin_example
[0]:root@guix03:~# man -k sbcl
sbcl: nothing appropriate.

[16]:root@guix03:~# man -w sbcl
/gnu/store/093cjg5472s8j8hjzi2as61cs4w3lwrf-sbcl-2.4.0/share/man/man1/sbcl.1.gz

[0]:root@guix03:~# manpath
/root/.guix-profile/share/man:/run/current-system/profile/share/man:/gnu/store/gjsxzcc0gqpz4lpbsrbidlnn5ij1lfm1-gzip-1.12/share/man:/gnu/store/g4wn6n3frwnjsay098mqgy046wxl4iym-coreutils-9.1/share/man
#+end_example

"MANPATH" environment variable is empty
#+begin_example
[0]:root@guix03:~# echo $MANPATH
#+end_example

Use `mandb' update the index database, with debug output
#+begin_example
[0]:root@guix03:~# mandb -d
 From the config file 
/gnu/store/w427xcp2628gg2wcxivvccw2pm2ijjk9-man-db-2.11.1/etc/man_db.conf:
   Mandatory mandir `/usr/man'.
   Mandatory mandir `/usr/share/man'.
   Mandatory mandir `/usr/local/share/man'.
   Path `/bin' mapped to mandir `/usr/share/man'.
   Path `/usr/bin' mapped to mandir `/usr/share/man'.
   Path `/sbin' mapped to mandir `/usr/share/man'.
   Path `/usr/sbin' mapped to mandir `/usr/share/man'.
   Path `/usr/local/bin' mapped to mandir `/usr/local/man'.
   Path `/usr/local/bin' mapped to mandir `/usr/local/share/man'.
   Path `/usr/local/sbin' mapped to mandir `/usr/local/man'.
   Path `/usr/local/sbin' mapped to mandir `/usr/local/share/man'.
   Path `/usr/X11R6/bin' mapped to mandir `/usr/X11R6/man'.
   Path `/usr/bin/X11' mapped to mandir `/usr/X11R6/man'.
   Path `/usr/games' mapped to mandir `/usr/share/man'.
   Path `/opt/bin' mapped to mandir `/opt/man'.
   Path `/opt/sbin' mapped to mandir `/opt/man'.
   Global mandir `/usr/man', catdir `/var/cache/man/fsstnd'.
   Global mandir `/usr/share/man', catdir `/var/cache/man'.
   Global mandir `/usr/local/man', catdir `/var/cache/man/oldlocal'.
   Global mandir `/usr/local/share/man', catdir `/var/cache/man/local'.
   Global mandir `/usr/X11R6/man', catdir `/var/cache/man/X11R6'.
   Global mandir `/opt/man', catdir `/var/cache/man/opt'.
   Global mandir `/snap/man', catdir `/var/cache/man/snap'.
   Added sections: `1', `n', `l', `8', `3', `0', `2', `3type', `5', `4', 
`9', `6', `7'.
path directory /root/.guix-profile/bin is not in the config file
   adding /root/.guix-profile/share/man to manpath
path directory /root/Projects/config/scripts is not in the config file
path directory /run/setuid-programs is not in the config file
path directory /root/.config/guix/current/bin is not in the config file
path directory /root/.guix-profile/bin is not in the config file
path directory /run/current-system/profile/bin is not in the config file
   adding /run/current-system/profile/share/man to manpath
path directory /run/current-system/profile/sbin is not in the config file
path directory /gnu/store/gjsxzcc0gqpz4lpbsrbidlnn5ij1lfm1-gzip-1.12/bin 
is not in the config file
   adding 
/gnu/store/gjsxzcc0gqpz4lpbsrbidlnn5ij1lfm1-gzip-1.12/share/man to manpath
path directory 
/gnu/store/g4wn6n3frwnjsay098mqgy046wxl4iym-coreutils-9.1/bin is not in 
the config file
   adding 
/gnu/store/g4wn6n3frwnjsay098mqgy046wxl4iym-coreutils-9.1/share/man to 
manpath
adding mandatory man directories
warning: /usr/man: No such file or directory
warning: /usr/share/man: No such file or directory
warning: /usr/local/share/man: No such file or directory
warning: /usr/man: No such file or directory
warning: /usr/share/man: No such file or directory
warning: /usr/local/man: No such file or directory
warning: /usr/local/share/man: No such file or directory
warning: /usr/X11R6/man: No such file or directory
warning: /opt/man: No such file or directory
warning: /snap/man: No such file or directory
final search path =
0 man subdirectories contained newer manual pages.
0 manual pages were added.
0 stray cats were added.
0 old database entries were purged.
#+end_example

"man -k" still can't find sbcl from its database
#+begin_example
[0]:root@guix03:~# man -k sbcl
sbcl: nothing appropriate.
#+end_example

Please enlighten me how to include guix manpages into mandb. Will there 
be improvement to guix's mandb to make it include manpages by default in 
guix's future release?

Thanks

Pan